Development of a simplified model for complex plastic waste gasification in an updraft fixed bed reactor

Development of a simplified model for complex plastic waste gasification in an updraft fixed bed reactor Matteo Tommasi a, Ilaria Prada a, Antonio Tripodi a, Ilenia Rossetti a a Dipartimento di Chimica, Università degli Studi di Milano, via C. Golgi 19, 20133, Milano, Italy, Chemical Plants and Industrial Chemistry Group matteo.tommasi@unimi.it Abstract (max 500 words) Worldwide, the amount plastic waste has doubled with respect to two decades ago and most of it still ends up in landfill, incinerated or dispersed into the environment, with only 9% of the total being successfully recycled. In the context of materials recycling, updraft gasifiers are promising small and middle-scale reactors to obtain building blocks and/or energy from plastic and biomasses. The latter can be mixed and reduced into pellets, while the needed heat enters the gasification process along with a heated carrier gas. In order to preliminarily design a gasifier to check its feasibility with an available feedstock, currently available models are inadequate. Thermodynamic ones are useless for the purposes of sizing, while too detailed rate-based models are too substrate specific are extremely time and resource consuming. The equipment scheme and the product yields are taken from available data [1], in order to validate the model predictions, while the reaction kinetics is derived for comparison with plastics [1–4]. This work simulates the combustion of Rofire pellets, a complex materials composed of mixed plastic-biomass fuel obtained by 55% from recycled plastics and 45% from biomass scrap by weight [1]. The same study was used to validate the model predictions [1]. A dynamic model is here presented of a fixed-bed reactor for biomass gasification, loaded continuously with down falling solid pellets and fed with counter-current air flow. Reactor structure is reported in Figure 1. The model considers: i) a one-step gasification kinetics, yielding a product spectrum matching experimental data from the literature (applying the kinetic parameters of polyethylene as a conservative scheme); ii) dynamic gas and solid energy balances and iii) stationary energy balance for the furnace. The model has been applied to describe a gasifier which consists of a vertical cylinder with an inner diameter of 0.4 m able to gasify 500 – 1000 kg/day of mixed wastes using air heated up to 1200 °C. The model is tested starting with an empty, cold furnace, increasing gradually the solid feed and the air flow and temperatures. The bottom stage solid outlet is closed to let out 2 – 3 kg/h of material (representing ashes) when the void fraction falls below 30%. The solid average temperature reaches a maximum at the end of the falling zone, where its thermal inertia is low. In the filled zone the material accumulation, together with the reaction duty, tends to ‘cool’ the pellets with respect to the fixed heat received from the gas. On the basis of the produced chemicals, the energy consumption was estimated as ca. 2 MJ per kg of solid feedstock. This simplified approach proves robust in describing the overall yields and start-up dynamic, showing higher reliability than equilibrium models in addressing the temperature profiles, at the cost of a simplified reaction kinetic and pellet description with respect to more complex simulation. The model validation was done by comparison between the calculations we performed and pilot-plant data. It can be concluded an overall good fit of the data. The solid-gas heat transfer and the bed packing are the main computational criticalities to achieve a reliable process description. Figure 1: (left) Multi-layers simulation scheme for the pilot-scale furnace. Each stage is assumed in perfect mixing conditions; (right) Detail of the flows connection for each simulation stage. References [1] A. Ponzio, S. Kalisz, W. Blasiak, Fuel Process. Technol. 87 (2006) 223–233. [2] A. Meng, S. Chen, Y. Long, H. Zhou, Y. Zhang, Q. Li, Waste Manag. 46 (2015) 247–256. [3] I. Mporas, P. Kourtessis, A. Al-Habaibeh, A. Asthana, V. Vukovic, J. Senior, Springer Proceedings in Energy Energy and Sustainable Futures Proceedings of 2nd ICESF 2020, n.d. [4] Y.A. Crespo, R.A. Naranjo, J.C.V. Burgos, C.G. Sanchez, E.M.S. Sanchez, WOOD FIBER Sci. 47 (2015) 9. Acknowledgments A. Tripodi gratefully acknowledges MUR for funding its RTDA position in the frame of the project Programma Operativo Nazionale “Ricerca e Innovazione” 2014/2020 to deliver research on “Green” topics. Re-cart Srl, Milan, Italy is gratefully acknowledged for support of this action.